During hydrocarbon cracking processes, carbonaceous deposits are formed on the reactor walls. Eventually, such carbonaceous deposits, if left to build to undesirable levels, can seriously restrict the flow of hydrocarbon vapors through the reaction zone vessel causing the pressure within the reactor vessel to increase to dangerous levels. Consequently, when a dangerous pressure level is reached, the reactor must be shut down. Many processes have been developed in the art of hydrocarbon cracking for dealing with this coking problem.
U.S. Pat. Nos. 3,557,241 and 3,365,387 disclose the introduction of sufficient steam and/or water to at least one tube of the cracking furnace while simultaneously reducing the hydrocarbon feed to that tube. The tube is then put back into service. The treatment of the tube is effected at temperatures ranging from as low as 370.degree. C. (700.degree. F.) to about 1100.degree. C. (2000.degree. F.). Such heat is supplied by external firing of the reactor tubes. Both Patents utilize a separate and distinct feed line for introducing steam and/or water for the so-called "on-stream decoking procedure". These lines are controlled by a valve which is put into service on only those occasions when the individual tube in question being decoked is undergoing such a cleaning operation.
While both Patents claim a multiplicity of tubes may be decoked at one time. U.S. Pat. No. 3,557,241, specifically states that it "contemplates the decoking of only a single tube at a time . . . " (Column 2. Lines 34-36), which is time consuming. Utilizing this method the furnace will be decoking during virtually all of its operational time. Furthermore, utilizing these two methods decoking a multiplicity of tubes at one time could cause a reduction in the production throughput of the system.
U.S. Pat. No. 3,920,537, deals with the coke deposition evolving from hydrocarbon cracking operations by "periodically contacting the coke deposit with a jet of relatively cold, high-pressure water." The Patent describes jetting the high-pressure cold water against the coke deposit in an amount sufficient to thermally shock and break up the coke deposit, typically at a pressure in excess of about 5000 pounds per square inch. This type of decoking technique, however, is only particularly useful where the coke deposition occurs on surfaces having temperatures of approximately 370.degree. C. (700.degree. F.) to 538.degree. C. (1,000.degree. F.).
German Patent Application No. 2923326 (See European Patent Application No. 0021167) discloses a method for decoking of equipment used in the thermal cracking of hydrocarbons which involves a two-step procedure utilizing steam and oxygen. The first step, involves conducting the gas flow of steam and oxygen through the equipment in an amount such that the temperature of the coke deposits on the heat exchanging surfaces of the cracking gas cooler are in the range of the prevailing thermocracking operating temperature. The second step involves intensifying the gas flow such that the temperature of the coke deposits on the heat exchanging surface of the cracking gas cooler is increased. Though this patent does involve a two step process, the second step merely involves the decoking of a separate piece of equipment e.g., the heat exchanger.
U.S. Pat. No. 4,203,778 effects decoking of furnace tubes by the use of a turbulent stream of impact resistant, non-angular, non-abrasive particles entrained in a gas stream. The particles are entrained at a concentration of 0.1 to 1.0 pound per pound of gas and the gas is introduced into the inlet end of the furnace tubes at a gas flow rate corresponding to an inlet velocity of 14,000 to 20,000 feet per minute.
The prior art decoking procedures in the hydrocarbon cracking field, operate under certain process constraints. The prior art utilizes decoking procedures wherein the reactors are made of metal. These processes are operated at reaction temperatures not exceeding about 1100.degree. C. Because the reactors are made of metal, the heat for the decoking reactors are transferred through the walls. They usually require taking the reaction train equipment out of service and specially treating that equipment so as to reduce or eliminate the coking problem. Furthermore, in most cases, these processes require the dismantling of equipment or the addition of equipment in order to effect decoking. Such procedures are exceedingly time consuming, and add materially to the cost of the operation of the hydrocarbon cracking apparatus.
There have been developed in the art processes for cracking hydrocarbons which utilize a flame cracking reactor. Such a flame cracking reactor system is depicted in U.S. Pat. No. 4,136,015. In particular, this patent refers to the "Advanced Cracking Reactor" (ACR) process. As characterized in said patent:
"In the `Advanced Cracking Reactor` (ACR) process, a stream of hot gaseous combustion products is developed in a first-stage combustion zone. The hot gaseous combustion products may be developed by the burning of a wide variety of fluid fuels (e.g. gaseous, liquid and fluidized solids) in an oxidant and in the presence of super-heated steam. The hydrocarbon feedstock to be cracked is then injected and mixed in a second stage zone into the hot gaseous combustion product stream to effect the cracking reaction. Upon quenching in a third stage zone, the combustion and reaction products are then separated from the stream."
The ACR process is described in varying detail in the following patents: U.S. Pat. Nos. 3,408,417; 3,419,632; 4,136,015; 3,674,679; 3,795,713; 3,855,339; 4,142,963; 4,150,716; 4,240,898; 4,321,131; 4,134,824; and 4,264,435.
In addition to the aforementioned Patents which are specifically directed to the ACR process, other Patents directed to the cracking of hydrocarbons by a flame-cracking process include U.S. Pat. Nos. 2,698,830, 3,565,970 and 2,371,147.
In the operation of such flame-cracking processes for converting hydrocarbons into more volatile components, it is necessary to effect the reaction in a reaction zone that contains a protective surface of a high-temperature resistant material which is also resistant to the products of the reaction. Illustrative of such materials are graphite, silicon carbide, alumina, zirconia, magnesia, calcium oxide and the like. All of these materials are extremely resistant to high temperatures but have low thermal conductivity. The continuous operation of the ACR process and a flame-cracking reaction process in general, causes coke deposition on the reactor walls. For example, U.S. Pat. No. 4,136,015 utilizes a reaction zone in which the stream therein is maintained at supersonic velocity flows. Coke formation in a system such as this, will materially alter the nature of the flows, thereby rendering the reaction process less controllable.
There is described herein a process whereby the coking problem can be effectively controlled and which circumvent the physical limitations of the aforementioned ceramic linings, i.e., low thermal conductivity. Furthermore this invention provides a method of decoking without alteration or dismantling of the reaction assembly.